Pop quiz: What happens if you pair 6 kW of modules with a 5-kW inverter? How much energy will be lost due to “clipping?”

We all know that the module rated power can be larger than the inverter rated power (within reason—inverters do have a max input current). But far fewer designers and engineers understand what are the practical limits.

The DC to AC ratio (also known as the Inverter Load Ratio, or “ILR”) is an important parameter when designing a solar project. For example, a 6-kW DC array combined with a 5-kW AC rated inverter would have a DC/AC ratio of 1.2 (6 kW / 5 kW = 1.2). The key driver here is the “clipping loss”: when the DC power feeding an inverter is more than the inverter can handle, the resulting power is “clipped” and lost.

We at Folsom Labs have found that many designers are overly conservative when thinking about DC/AC ratios. Many people think DC/AC ratios of 1.1 are ideal, with 1.2 as slightly aggressive. Instead, design values of 1.2 often result in minimal losses, while a 1.25 or 1.3 value can improve project economics, especially when a project size is constrained by the AC capacity.

Why and how do inverters clip?

Every inverter has a maximum rated power. This is important for two reasons. First, the component ratings of the power electronics in the inverter are often designed with a specific power and voltage range in mind. Second, at the system level, the home’s AC panel (and/or the grid connection point) are designed with a specific max power in mind.

Inverters will generally never output more than their max-rated AC power. During times when the DC input power is too high, the inverter will raise the operating voltage of the modules to pull the array off of its max power point and reduce the DC power.

Why a 20% DC/AC ratio results in minimal clipping losses

Many designs start with an assumption of a maximum 1.2 DC-to-AC ratio (in other words, 20% large module power rating versus the inverter max power rating). This actually keeps losses extremely low, usually under 0.25%. Why is this? Three factors help explain these low losses:

1. Full “standard” conditions (1,000W/m2) are rare.

A module is rated at “Standard Test Conditions” (STC), which is sunlight of 1,000W/m2 (basically noon on a summer day). In practice, systems rarely receive these idealized conditions.

For example, the chart below shows a frequency chart for a solar array in Atlanta facing south at a 15° tilt. Note that the array rarely sees full sunlight. In fact, just 422 hours (9% of the operating hours) see more than 800W/m2 (equivalent to clipping at a 1.25 DC/AC ratio).

2. Temperature losses reduce the high-power times even further.

In addition to the fact that the array rarely gets full sun, there are other system losses between the module surface and the inverter. In particular, temperature losses make a huge difference here: modules are typically hotter than 25ºC, particularly when the array is receiving maximum sunlight. Therefore, the array produces less than the rated power, and it doesn’t reach over-power conditions at the inverter.

Back to the Atlanta example: Let’s look at how often the modules are producing close to their rated power. Note that the top end of the distribution is even thinner because this data also includes the temperature losses, which will be greatest at the times of highest sunlight. This time, there are just 212 hours (4.5% of the operating hours) when the modules are producing over 80% of their rated max power.

3. Production does not go to zero when the DC power is greater than max AC power.

Generally, when an inverter is in over-power mode, it simply means that it will sacrifice the excess power. So even when the actual DC power is 10% over the max AC power, the losses are just 10% for that time.

Looking to the Atlanta example again: For the 212 hours when the modules are producing greater than 80% of rated power (the cutoff point for a 1.25 DC/AC ratio), the average power is just 6.8% over the limit. This effect is even more extreme with residential arrays. Because residential systems are flush-mounted, they run hotter, and therefore the temperature losses are even larger. As a result, the Atlanta system with a 1.25 DC/AC ratio has total clipping losses of just 0.6% for commercial arrays, and 0.1% for residential arrays.

Digging in more: different DC/AC values

Since an ILR of 1.25 in Atlanta represents just 0.1% to 0.6% clipping losses, this raises a question: What would the losses be if we increased DC/AC ratios more dramatically? See below:

Clipping losses are zero for DC/AC ratios of 1.15 in both system types. In commercial arrays (with lower temperature losses), the module power can go up to 365W while still keeping clipping losses under 2%. Finally, residential arrays can go up to 380W with clipping losses under 1%.

Conclusion

The next time someone tells you that a rated DC power is too large for an inverter, check it out for yourself. You may find that when you model the system production, the clipping losses are lower than you expect.

About The Author

Paul Grana

Paul Grana is the co-founder of Folsom Labs, where he leads sales and marketing, where he has helped to grow the company to thousands of installers in over 70 countries. He also founded the S3 Solar Software Summit, which brings together the industry’s leading software vendors and buyers each year. He previously worked at Abound Solar, and led product management and technical marketing with Tigo Energy. Paul holds a BS in Mathematics and Economics from the University of Chicago, and an MBA from Harvard Business School.

Excellent article, provided great insight into clipping losses, but as stated under “Why a 20% DC/AC ratio results in minimal clipping losses” the DC/AC ratio is the ratio between the module power rating and inverter max power rating. Would this still be the case if the inverter controllers power factor was not at unity i.e. (p.f. 0.8); Or would the DC/AC ratio be set on the MWac output with 0.8 power factor? i.e. (0.8 x Inverter max power rating)

Hi Thanks for the great article I just was trying to understand why the designer put a 6kw sma with 8.1kw solar and I think this explains it. I installed this for another company and was concerned. I normally design to 1.1 or 1.2 and was fearful this would cause a failure. So 1.35 should just clip and not cause issues with the inverter itself correct this is a 6kw sma I have not worked with SMA i the past and do not know the track history on them. I have used many others. I do know the voltage is fine though.

I have seen clipping on my inverter and output reductions due to the inverter reaching 150 degrees. I had the inverter put in a cool basement because the alternative was on a very hot south facing wall next to the AC condenser. The installer said I should put a cooling fan on it due to potential overheating. He proves right. I was upset with them over installing a 3800 watt inverter on 4350 watt array. I live at 3000 feet in inland Southern California and sun is the norm a lot of the time. I was taught to over engineer things and this is blatant under engineering for the sake of money. And to see a solaredge go along with this makes me question their integrity.

Inverters are most efficient when running at or near full capacity. Going up an inverter size, for example installing an SE-10,000 inverter with a 7,500-watt system, would make the system less efficient. The lost production in that scenario would be more than the little bit of production lost from occasional clipping. Put another way, the choice is lower efficiency 365 days a year, or very high efficiency year-round with occasional clipping. You get considerably more solar production in the second scenario. Did you read the article? Your DC to AC ratio 1.14. If a SE 5000H was used your DC to AC ratio would be .87. Do more research. You installer did not under engineer for the sake of money. They were trying to design you a more efficient system as the article states.

If you have a 6kW DC array (20, 300W Itek panels). Wouldn’t it make sense to use 6KW of AC microinverters (10 AP Systems- 600 watt AC for 2 panels) , especially if the cost of the 6kW microinverters is almost identical to the cost of 5kW microinverters? Would the 1:1 ratio assure no clipping losses at all?

what will be the response of grid tied solar inverter if load exceeds the output power of the inverter.for example if i install 10 kw inverter for my office and suppose my load reaches to 15 kw ,so what inverter will do in this situation.

Your inverter’s name plate rating is 10K You probably won’t produce 10K unless you have a super savy designer & installer and in that case you will still most likely reach 95-98% of the name plate. Check the AC rating! Also what your Load is, building electrical demand, has nothing to do with your inverters production at max let alone what your system is designed to provide. you may need an upgrade to a larger system. Or just add some micro inverter units as your needs dictate.

GHI and on tilt area, ambient temp, NOCT, ambient temperature around inverter (or temperature triggered ventilation fans) – all of these need to be cumulatively calculated for the whole year and it will vary with each of the above parameters. There are some areas of concern when it comes to economics of reducted no. of inverters versus reliability of inverter (most active components break at high voltage (snubber circuits in use?), current density at high junction temperatures).

Also, an inverter designed for high DC/AC ratio is possible with some margin cooling designs say for a high temperature tropical country. Good news is that at high ambient temperature, the DC output has already reduced.

Something my company have found problematic is the variation in the amount of clipping from year-to-year.
If the performance KPI in your contract is PR you can find yourself in a high-yield/low-PR situation relative to the design year as sunnier years have higher levels of clipping which of course reduces the PR.

We believe the solution is to guarantee a power curve vs POA irradiance, or even a power surface against POA irradiance and module temperature but it takes time to bring clients round to a new choice of KPI.

Is there another word in the solar panel context that can be used instead of clipping, such as underutilization, or unrecoverable power?
In electronics, clipping is what happens when an amplifier or inverter cannot amplify beyond the hard limit of the power supply or when an analog to digital converter cannot store the sample in the number of digits available, so the peaks and troughs are lopped off and produce distortion and high frequency harmonics. This is clipping.

Having a larger array may exceed an inverter’s demand by a couple of percent of the time, but because this is more than compensated for by the 99% of the time that the panels are generating in lower light levels, is “clipping” really a loss at all?

Don’t forget about east / west arrays in commercial and multi- facet arrays in residential. Inverters only clip when at max current output. If you get high morning output for panels facing east and then later, high afternoon output for panels facing west, you will see reduced clipping. This is why we go up to 1.35x on all SolarEdge residential and commercial inverters.

Dual Azimuth systems are great and you could probably move the ratio up as far as 1.45 with considerations to roof pitch, 90 degree offset of azimuth, etc. Fo an East or West facing array to reach 80% it would take a higher than normal altitude, late spring with cool temps cool temps (module to exceed name plate slightly) , super clear upper atmosphere (jet stream blown clean). The benefit of a mixed array is always that unless you really push the the ratio it never reaches full potential but offer a long flat Max Power Curve.

We have struggled with this ideal clipping loss issue for years. One thing that keeps coming up is we get a lot more sunlight in New Mexico than places like Atlanta. Every time I bring up clipping, the attitude is that we already clip at 1.0 due to abundant 1200 w/m^2 days. Only recently has that been challenged and we are digging into the details. But what may work for other areas of the country does not always fit here.